Sftp support

This adds support for the SFTP protocol. 
It supports downloading files, and uploading files. Uploaded files are saved in the downloads directory.
It gets accurate ownership, permissions and sizes of files from the pickle file. 

The 'fs' module got additional functions that mimic standard UNIX fs interface.
The 'ssh' module got SFTP protocol support and calls the fs functions.

SFTP support is explicitly disabled unless turned on in the config file.